1. Field of the Invention
The present invention relates to the technical field of wireless transmissions and, more particularly, to a Doppler frequency estimation system and method.
2. Description of Related Art
The wireless transmission channels are dynamically changed with a relative motion of a transmitter and a receiver. The statistic of time-varying features for a wireless transmission channel is highly dependent of a Doppler spectrum. The Doppler spectrum is obtained by performing a Fourier transformation on the autocorrelation function of a channel pulse response, also referred to as a Doppler spread. The Doppler spread is positively proportional to the relative motion speed between a transmitter and a receiver.
The channel's features are varied over time due to the Doppler effect. This increases the uncertainty of the signal quality. In addition, the Doppler spread causes a frequency offset at the receiver to thereby increase the bit error rate (BER) of the receiver. Thus, estimating the Doppler spread and the motion speed at the receiver can directly influence the performance of a mobile communication system. For example, for code division multiple access (CDMA) systems such as IS-05, WCDMA and CDMA2000, the motion speed at the receiver is used as a reference of the important parameter of switching a mobile or cell phone system. In orthogonal frequency division multiplexing (OFDM) systems, accurately estimating the Doppler spread and the motion speed at the receiver can benefit the synchronous and time-varying channel estimations at the receiver.
U.S. Pat. No. 6,563,861 granted to Krasny et al. for a “Doppler spread estimation system” has disclosed a method of using a Fourier transformation to directly estimate the maximum bandwidth of a Doppler spectrum. FIG. 1 is a schematic diagram of a direct maximum bandwidth estimation typically using a Fourier transformation. As shown in FIG. 1, a multiplier 26 receives the sampled received signals rn and multiplies the signal by the transmitted symbols {circumflex over (d)}*h to thereby produce a corresponding autocorrelation function which is subsequently passed through a low pass filter (LPF) 28 to thereby eliminate the high-frequency noises. A second processing block 30 performs a fast Fourier transformation (FFT) to change the autocorrelation function to be in frequency domain to thereby produce a signal spectrum. A third processing function 32 is a multi-channel correlator, and each channel of which performs a correlation operation in frequency domain on the bands of the signal and the Doppler spectra to thereby produce a likelihood ratio metric. A maximum function block 34 selects the frequency corresponding to a maximum likelihood ration metric as an estimate of actual Doppler frequency.
Such a technology mentioned above can directly determine the estimate of actual Doppler frequency. However, due to a limit of the motion speed at the receiver, a Doppler frequency generally ranges from a couple of 10 Hz to 1.5 KHz. For accurately estimating the Doppler frequency, the second processing block 30 performs the FFT under a higher resolution requirement. Namely, the data amount to be processed in frequency domain becomes more and thus the entire system cost is increased. However, the data amount is reduced as the resolution is reduced on the FFT performed by the second processing block 30, but the Doppler frequency at the receiver cannot be accurately estimated, resulting in negatively affecting the system performance.
Therefore, it is desirable to provide an improved Doppler frequency estimation system and method to mitigate and/or obviate the aforementioned problems.